That's nice. I figured out what was going on (though I was very frustrated by that point) and switched my DNS servers to Google's public DNS servers (8.8.8.8 & 8.8.4.4) which worked fine. I figured that calling would be a waste of both my time and Comcast's - the people on the other end of the phone have no idea how long a fix is going to take other than 'as soon as possible'.
The straightforward explanation and humble acceptance of responsibility were a refreshing surprise, however. A $5 credit is not that big a deal but it is good to see a firm putting its money where its mouth is instead of trying to deflect or minimize the disruption to customers.